The present invention generally relates to an imaging lens that forms an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. More particularly, the present invention relates to an imaging lens to be mounted on a relatively small camera, such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an in-vehicle camera, and a network camera.
In the recent years, cameras are included as a standard feature in most of cellular phone, so that the values added to those cellular phone are increased. The integration of the cellular phone and the digital still cameras has been developed year after year to a point where some cellular phone are equipped with cameras having optical performance and various functions comparable to those of digital still cameras. However, the principal function of the cellular phone is still a communication function, and, even today, those cellular phone are naturally expected to have basic characteristics such as a small size and a light weight.
In response to the trend of the size and weight reductions in the cellular phone, there has been a demand for smaller imaging lenses to be mounted on the cellular phone. Conventionally, sufficient optical performance in accordance with the resolutions of imaging elements and miniaturization have been both realized with imaging lenses having a two-lens or three-lens configuration. However, required optical performance has become higher every year in line with higher resolutions of imaging elements, and it has become difficult to realize high optical performance with appropriately-corrected aberrations and miniaturization at the same time depending on a two-lens or three-lens configuration.
For this reason, there have been studies in adding another lens, i.e., an imaging lens with a four-lens configuration. For example, an imaging lens disclosed in Patent Literature 1 is formed with a first lens that has a convex surface on the object side and is positive, a second lens having a negative meniscus shape with a concave surface facing the object side, a third lens having a positive meniscus shape with a convex surface facing the object side, and a fourth lens having a positive meniscus shape with a convex surface facing the object side, arranged in this order from the object side.
In such a configuration, preferred ranges are set for a ratio of a focal length of each of the first to the third lenses to the focal length of the lens system, for the refractive index of the first lens, and for the Abbe number of the first lens. Those values are restricted within the preferred ranges, so that it is possible to realize preferable optical performance while restraining an increase in a length of the imaging lens.
Citation List
    Patent Literature 1: Japanese Patent Publication No. 2007-122007
With the above described imaging lens disclosed in Patent Literature 1, relatively preferable aberrations can be obtained. However, the cellular phone has been becoming smaller and highly functional every year, and there is a demand for very small imaging lenses.
With the lens configuration disclosed in Patent Literature 1, it is difficult to satisfy such a demand and realize imaging lens miniaturization and appropriate aberration corrections at the same time. It should be noted that simultaneous realization of miniaturization and appropriate aberration corrections is not a challenge only for the imaging lenses to be mounted on the cellular phone, but is a common challenge for the imaging lenses to be mounted on relatively small cameras such as digital still cameras, portable information terminals, security cameras, in-vehicle cameras, and network cameras.
The present invention has been made in view of the above described problems in the conventional art, and an object thereof is to provide an imaging lens that can appropriately correct aberrations though a small size configuration.